Re: [dpdk-dev] [PATCH v2] mk: report address of packed member as warning

[Bruce Richardson <bruce.richardson@intel.com>]

On Thu, May 02, 2019 at 04:24:37PM +0200, David Marchand wrote:
>    On Thu, May 2, 2019 at 4:13 PM Reshma Pattan
>    <[1]reshma.pattan@intel.com> wrote:
> 
>      gcc 9 on Fedora 30 gives an error
>      "taking address of packed member may result in an
>      unaligned pointer value" for -Waddress-of-packed-member.
>      Report it as warning instead of error to fix the build.
>      Snippet of build before fix
>      ...lib/librte_eal/linux/eal/eal_memalloc.c: In function
>      ‘alloc_seg_walk’:
>      ...lib/librte_eal/linux/eal/eal_memalloc.c:768:12: error: taking
>      address
>      of packed member of ‘struct rte_mem_config’ may result in an
>      unaligned
>      pointer value [-Werror=address-of-packed-member]
>        768 |  cur_msl = &mcfg->memsegs[msl_idx];
>            |            ^~~~~~~~~~~~~~~~~~~~~~~
>      Snippet of build after fix
>      ..lib/librte_eal/linux/eal/eal_memory.c: In function
>      ‘remap_segment’:
>      ..lib/librte_eal/linux/eal/eal_memory.c:685:9: warning: taking
>      address
>      of packed member of ‘struct rte_mem_config’ may result in an
>      unaligned
>      pointer value [-Waddress-of-packed-member]
>        685 |   msl = &mcfg->memsegs[msl_idx];
>      Signed-off-by: Reshma Pattan <[2]reshma.pattan@intel.com>
>      ---
>       mk/toolchain/gcc/[3]rte.vars.mk | 4 ++++
>       1 file changed, 4 insertions(+)
>      diff --git a/mk/toolchain/gcc/[4]rte.vars.mk
>      b/mk/toolchain/gcc/[5]rte.vars.mk
>      index d8b99faf6..61032bbbc 100644
>      --- a/mk/toolchain/gcc/[6]rte.vars.mk
>      +++ b/mk/toolchain/gcc/[7]rte.vars.mk
>      @@ -87,5 +87,9 @@ WERROR_FLAGS += -Wimplicit-fallthrough=2
>       WERROR_FLAGS += -Wno-format-truncation
>       endif
>      +#Report "taking address of packed member may result
>      +#in an unaligned pointer value" issues as warnings.
>      +WERROR_FLAGS += -Wno-error=address-of-packed-member
>      +
>       export CC AS AR LD OBJCOPY OBJDUMP STRIP READELF
>       export TOOLCHAIN_CFLAGS TOOLCHAIN_LDFLAGS TOOLCHAIN_ASFLAGS
>      --
>      2.21.0
> 
>    No, you can't do this unconditionally.
>    Looked at clang (in make build system) and meson (globally) and those
>    completely disable the warning.
>    $ git grep address-of-packed-member origin/master --
>    origin/master:config/meson.build:       '-Wno-address-of-packed-member'
>    origin/master:mk/toolchain/clang/rte.vars.mk:WERROR_FLAGS +=
>    -Wno-address-of-packed-member
>    It should be something like (untested):
>    @@ -87,5 +87,10 @@ WERROR_FLAGS += -Wimplicit-fallthrough=2
>     WERROR_FLAGS += -Wno-format-truncation
>     endif
> 
>    +# disable packed member unalign warnings
>    +ifeq ($(shell test $(GCC_VERSION) -ge 90 && echo 1), 1)
>    +WERROR_FLAGS += -Wno-address-of-packed-member
>    +endif
>    +
Actually, you should not need to check the GCC version for this.
GCC always ignores silently any warning disable flags it doesn't recognise,
only printing information about them if there is another error to report.
[This is why meson, when you ask it if the compiler supports "-Wno-foo", will
check the flag and also the -Wfoo flag, as gcc won't report an error with
just the "no" form]

/Bruce